It is known to use linear charged coupled devices (CCDs) to scan colored objects such as documents or film. The CCD can be used in either focal plane or contact optical configurations, and in reflection or transmission modes of operation. The video line-scan signal of the CCD when measured produces the pixels of the digital image in one dimension (referred to as the "line-scan"). Through relative motion of the object-to-CCD perpendicular to the line-scan direction, multiple lines are allowed to be measured. This produces the pixels in the page-scan direction.
In order to scan several objects automatically, many scanners are known that utilize one mechanism to advance and position the next object to be scanned, and a second mechanism to produce the page-scan motion with respect to the CCD detector. In these known scanners, the page-scan mechanism must reciprocate, i.e. it may scan in a left to right or clockwise direction and then retrace its motion to prepare for the next scan. The requisite retracing of the page-scan mechanism is of no use to the scanning operation and slows down the scanning process.
Other known scanners use only a single mechanism to produce both the page-scan motion and the advance to the next object in order to scan several objects automatically. For such scanners with continuous feed of the object, for example, a capstan film drive or pinch rollers through which are moved individual document pages, the scanning motion for the next object can begin as soon as that object is moved into position. Such a system provides little wasted motion. However, if one wishes to preview scan before performing the full scan, e.g. perform a low resolution scan first and then a high resolution scan, the object must be rewound or repositioned to perform the second scan. Again, this slows down the scanning process.
The ability to provide low or "reduced" resolution scanning is available in many known commercial scanners. Fundamentally, it is performed through a resampling of the digital data. For example, in several desktop scanners that are currently available, it is a common practice although somewhat crude, to average groups of "n" pixels in the line-scan direction, thus leaving (1/n)x fewer pixels per line. In the page-scan direction, the sampling pitch, i.e. the distance between successive line measurements, is then increased. Such a simple pixel-summing method is well known to those having ordinary skill in the art.
In order to allow a multi-channel image, such as a color representation, to be digitized in a single pass of the scanner, tri-linear CCDs have been used to scan the object. A tri-linear CCD has three physically separate, parallel and equally spaced linear arrays of photosites integrated onto a single device such as a silicon chip. However, because each of the three linear arrays are distinct from one another and therefore cannot be arranged coincident with each other, the separation between the line arrays results in channel-to-channel spatial delay of the received image data in the page-scan direction.
In order to simplify the processing of the received image data, the channel-to-channel spacing is designed to be an integer number, N, times the line-scan pitch, P.sub.1, i.e. the distance between the photosites along the linear array. By restricting the product of the page-scan sampling pitch, P.sub.p, times an integer number M, to be equal to the original channel-to-channel distances: EQU P.sub.p .multidot.M=P.sub.1 .multidot.N for M=N, N-1, N-2, . . . 2, 1.
the channel-to-channel spatial delay introduced will be an integer "line" amount of the value M, and is therefore easily removed during processing.
Also, the channel-to-channel spacings are designed in a tri-linear CCD to be equal. For example, for a red- green-blue (RGB) tri-linear CCD, the spacing between the red array and the green array, i.e. the red to green line delay, is equal to the spacing between the green array and the blue array i.e. the green to blue line delay, and is twice that of the red-to-blue spacing.
Known solutions to "re-align" the image data from each linear array in a tri-linear CCD have attempted to place the image data into a buffer large enough to hold the entire image. The image data is then read out of the buffer in a line-delayed manner. Such an approach is very costly due to the fact that the buffer must be larger than necessary to remove the channel-to-channel delay. Also, the use of a large buffer is very time consuming because one must wait until the image data is aligned before it can be color processed.
The use of an analog line delay with a multi-CCD device is also known. By changing the clocking of the line delay structures, one can vary the amount of delay and the channel to which it is applied. However, such analog delays add noise and have dark signal offset voltages which will vary with delay time. This presents problems in that the calibration of the device changes with its operating mode.
In view of the deficiencies with these known scanners, there is therefore needed a scanner having a scanner control circuit which provides digital real-time spatial delay correction of tri-linear CCD scan image data to allow color bi-directional scanning with varying spatial resolutions.